Excellent Question!

The short answer is that prior to the early 18th century, the name "Thirty Years War" (Dreißigjähriger Krieg) is hardly seen at all. The Thirty Years War was a not a single straightforward war with only two belligerents, but a series of inter-related conflicts across several theaters in both Western and Central Europe. At the time the wars were ongoing, there was little sense that it was single conflict with a identifiable set of participants. Contemporary accounts simply call it "the current wars" or "the wars in the Germanies" (note the plural!)